Wireless communication systems are known to include a plurality of wireless communication devices that communicate directly (e.g., point-to-point) or through an infrastructure. For direct communications, a wireless communication device, such as a radio, cellular telephone, station coupled to a personal computer or laptop, et cetera, transmits data on a particular radio frequency channel directly to another wireless communication device. For infrastructure-supported communications, a wireless communication device transmits data on an assigned radio frequency channel to an access point (or a base station). The access point determines the targeted wireless communication device from the received RF signals. If the targeted wireless communication device is affiliated with the access point, the access point transmits the data to the targeted wireless communication device on a radio frequency channel. If the targeted wireless communication device is not affiliated with the access point, the access point forwards the data to a central station, which routes the data to the access point that is affiliated with the targeted wireless communication device.
To ensure reliability of data transmissions within a wireless communication system and to ensure interoperability of differing manufacturers' equipment, standards have been developed. Such wireless communications standards include IEEE8 02.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wireless application protocols (WAP), local multi-point distribution services (LMDS), multi-channel, multi-point distribution systems (MMDS), and/or variations thereof.
Such standards prescribe operating parameters for a particular type of wireless communication system. For example, the IEEE 802.11a standard defines a wireless local area network that prescribes a frequency band of use, division of the frequency band into channels and sub-channels, encoding/decoding convention, modulation/demodulation convention, frame format, data rates, et cetera. For instance, the IEEE 802.11a standard provides various combinations of data rates and modulation schemes, which can be selected via a coding rate corresponding to a particular modulation scheme.
As is known for wireless local area networks, wireless communications devices roam within the coverage area of the wireless local area network. As such, signal strength of radio frequency signals to and from a wireless communication device varies depending on the interference between the wireless communication device and another wireless communication device or the access point and the distance therebetween. In accordance with the IEEE 802.11a standard, data rates and modulation schemes may be adjusted based on signal strength and/or interference of RF signals. For example, if the signal strength is strong and there is minimal interference, the communication may be done at 54 Mbps (megabits per second) using a 64 QAM (quadrature amplitude modulation) modulation scheme. If, on the other hand, the signal strength is weak and/or there exists substantial interference, the communication may be done at 6 Mbps using a BPSK (binary phase shift keying) modulation scheme.
The various combinations of data rates and modulation schemes prescribed by the IEEE 802.11a standard provides adequate granularity of data rates for typical data transmissions (e.g., email, file transfers, and internet access) for wireless communication devices that move within the local area network. However, for relatively stationary wireless communication devices that transmit and/or receive video data, the granularity of data rates provided by the IEEE 802.11a standard may not be sufficient. For example, the IEEE 802.11a standard provides a 24, 36 and 48 Mbps data rates, which are spaced at 12 Mbps. Thus, if a wireless communication device barely fails to support a 48 Mbps rate it drops to a 36 Mbps data rate. For MPEG video streams, which have a bandwidth of about 2 Mbps, the change from 48 Mbps to 36 Mbps reduces the number of video streams that a channel can support by 6. Such a loss of potential video streams on a particular channel in many applications is highly undesirable.
Therefore, a need exists for a method and apparatus that provides for greater granularity of standardized data rates in a standard compliant wireless communication system.